The invention deals with a device for electroerosion with a piece of electrode by means of an electrode wire stretched and unwound between two wire guides. In their simplest forms, such devices permit cutting a piece longitudinally in a trajectory situated in a plane called the principal plane perpendicular to the stretched wire between the guides. This allows for a right angle; otherwise, in general; cutting of all the surfaces generated are perpendicular to the principal plane.
In order to obtain more general forms, i.e. pieces to be machined, etc.; it is necessary that the wire should incline in relation to the normal principal plane: the neutral axis. It follows that the piece is tilted in relation to the plane in order to machine finish it. Many methods have been proposed to accomplish the first alternative. Three types can be distinguished:
(a) Displacement of the guides in relation to the other, parallel to the principal plane, combined with the rotation of the guides around a parallel axis to this principal plane as described by U.S. Pat. Nos. 4,431,896, 4,460,816, 4,461,942 and 4,453,070 and German Pat. No. 1,615,508 or in EP Pat. No. 201.911. This displacement could be obtained by means of a different apparatus; a table of cross-movements; hydraulic apparatuses that translate according to combined axes and to rotating tables. etc . . . ;
(b) A tilting according to `Cardan` of the whole ensemble formed by two guides around two axes of rotation situated in the plane of the cutting trajectory, such as described in U.S. Pat. No. 4,485,288 or in German Pat. DE No. 2,413,627.
(c) A combination of rotation of guided wires around an axis of parallel rotation to a neutral axis with displacement of one of the guides in relation to the other, parallel to the principal plane, as an example, U.S. Pat. Nos. 4,123,645 and 4,420,671.
In practice, the second method which is the only one which preserves the length of the stretched electrode wire between two guides whatever the angle of inclination; demands an increase in free space so rotations can be accomplished without collisions, and in particular the apparatus fixes the piece to be tooled, or machined, or finished. This proves to be difficult for obtaining precision when machine finishing by electroerosion.
The first method presents equal inconveniences in regards to the phenomenom of incumbrance, in terms of machine finishing at a satisfactory speed, and approaching each other as much as possible. The heads of the machine finishers must carry the guide wire on the surface of the piece to be machine finished. This small space situated inbetween the piece and the outer limit of the heads of the machine doesn't permit the inclination of an important angle of the heads to be machined, and therefore, the wire electrode. One is limited to the smaller angles of stripping or machining.
Therefore one of the goals of the current invention is to finish with great `strippage`, that is to say to effect the cuttings of a piece according to the surfaces of a strong inclination of the angle of the wire electrode, with the neutral axis being important: i.e. more than 30.degree.. This is because the present invention has reference to a third method. This is for an object for machine finishing by electroerosion with a wire electrode. This is by means of the stretched wire electrode wound between two wire guides with at least one device with crossed movements carrying these guide wires and adjusting to the displacement in a plane parallel to the cutting trajectory, in such a fashion as to incline the wire to a predetermined angle in relation to a neutral axis perpendicular to this plane. This is characterized by this device, and is equally adjustable in order to be able to pivot the wire guide over itself around an axis parallel to the neutral axis, passing by the guide and displacing itself with it. It can also be characterized by an adjustable device in order to subjugate the wire guide to these placements of the device, to crossed movements.
As one projects to machine finish with great angles of inclination, the closed guides present certain inconveniences in part because of their limited precision. By closed guides one understands it is that by having in general; of the type, a symmetry of rotation of coils, or of the type of guide wires, guide barrels, or shanks; or of the type, as an example, described in U.S. Pat. No. 4,205,212. This deficiency in precision in particular, is due to the friction created with the advancement of the wire, and by constraining it. The latter occurs when it is strongly inclined in relation to the axis of the guide and its rigidity. There are increases in risks of breakage and conductivity. In order to decrease the speed of the machine finishings, therefore, one must decrease the speed of the rate at which the machine finishes by one half; for example when one finishes with an angle of 30.degree. or more. Given on the other hand the encountered difficulties with threading and rethreading the wire in such guides, the machine according to the present invention holds the advantage for open guides.
These guides can have all kinds of configurations as described by U.S. Pat. Nos. 4,123,645 or 4,420,671. In general they present a gap or peripheral groove where the wire lodges according to the given profile of guidance. It could simply be a device with a uniquely pivotable small pulley, or small wheel, around a parallel axis; a neutral axis for a wheel or cylinder. This is pivotable around an axis parallel to a neutral axis, and passing preferably by their centers of symmetry, and pivotable also around their axes of revolution provided with a gap for guidance of the wire. The plane for this gap is preferably perpendicular to the axis of revolution. These guides can be conductors of electricity and serve to feed the wire standard current for its functioning.
The device of the current invention presents notable advantages to American patents, U.S. Pat. Nos. 4,123,645 and 4,420,671. In the present invention better precision is possible with the table with crossed movements rather than blocks which require manual direction or intervention, and turning disks which are described in U.S. Pat. No. 4,123,645; or with coupled screws with a rotating device as described in U.S. Pat. No. 4,420,671. Another disadvantage of U.S. Pat. No. 4,123,645 is the form of the guide wire which necessitates a disservice of orientation, in such a fashion as to maintain the plane of the gap within a predetermined angle with the direction of the cutting, in order to stabilize the electrode wire within the gap of the guide and to ensure the preciseness of the cutting. At the same time a simpler form of the wire guide in the current invention is an advantage to the coupled screw guides described in U.S. Pat. No. 4,420,671.
Above all, one can easily see that the construction of the present invention permits the positioning of the guides with all the necessary precision, without demanding the devices positioning of the guides in extremely low tolerances. In effect, where the wire guide (or guides), pivots on itself, that means that the parallel axis to the neutral axis around which this guide rotates, (passes evidently by preference) by its center of symmetry and moves with it in its displacements in a plane parallel to the principal plane. Consequently the horizontal shifting or unwedging, between this axis and the point where the wire leaves the guide, also rests weakly even when the wire is strongly inclined. The influence on the precision of the orientation of the guide(s) on the positionment at this point determines the exact position of the wire which therefore, remains equally weak. What simplifies this so much in the current invention is the mechanical realization of the device to orient the guide(s) and reduce crowding to acceptable values.
The member in the device is described in U.S. Pat. Nos. 4,123,645 and 4,420,671. The axis of rotation is fixed and passes through the center of turning plates on which the guides are fixed. It is therefore confused with the neutral axis when the guides are altered or shifted in a plane parallel to the principal plane in order to incline the wire. They cannot therefore pivot on themselves, but they affect a rotation around a neutral axis.
Another advantage to the present device is that it permits the orientation of the guides not to form a given angle with a trajectory to cut as it is described in U.S. Pat. No. 4,123,645 but in such a fashion that the planes of each of the profiles of guides, (that is to say that the trajectories imposed on the wire around the wire guide) should be coupled and contained as a part of the stretched wire between the two guides. This means that the wire doesn't rub strongly and it facilitates its gliding into the guides reducing the risk of its losing its place, and prevents the alteration of mechanical tension where there are changes in the angle of inclination.
Such an orientation was not possible with the known mechanisms:
In U.S. Pat. No. 4,123,645 the planes of the profiles of guidance are strongly parallel to the guide wire and have to be oriented in this fashion to make a given angle with a trajectory. They cannot be confused, except it the displacement of the destined guide that inclines the wire in relation to the neutral axis has been effected according to a single situated axis in the plane of the profile directing this guide. That is what limits the possibilities.
In U.S. Pat. No. 4,420,671 the wire guide are cylinders with a screw thread inclined in relation to the principal plane in order that the plane of the guides should be perpendicular to the principal plane. It is the rotation of each of the guides around their axes of revolution that make the wire advance in the direction of the axis. The axis of revolution of the two guides are crossed. The planes of the profile of guidance can never be confused nor can they contain a part of the stretched wire between the guides.
Furthermore, the invention evades the shifting or pushing of the guide around a parallel axis in the plane of the cutting trajectory of orientation. This simplifies construction and permits surrounding of the guide with a covering forming a nozzle for injection of the liquid that is used. This covering will not require a shifting around a horizontal axis, and will be more easily conformed in this way, so that the mouthpiece of the nozzle rests constantly in the immediate proximity of the piece to be fabricated, which is a vital condition for the speed and positioning of the machining.